The use environment of structural steel materials, such as ships, offshore structures, or the like, or thick steel plates for multipurpose tanks storing various kinds of liquefied gases, such as carbon dioxide, ammonia, LNG, or the like is very severe. Therefore, the strength of such steel sheets is very important. To enhance strength, techniques that may enhance the hardness and strength of steel sheets by adding an hardenability enhancing element to form a low-temperature transformation phase within the steel sheet during the cooling thereof have been proposed.
However, when a low-temperature transformation phase, such as martensite, is formed inside steel sheets, toughness of the steel sheets may be severely deteriorated due to residual stress contained therein. That is, strength and toughness of steel sheets are two physical properties the compatibility of which may be difficult to realize, and it is generally understood that when the strength of steel sheets increases, the toughness thereof decreases.
In the case of the steel materials for offshore structures or the steel materials for tanks, the toughness thereof at low temperatures, as well as the strength thereof, is very important. First of all, environments in which steels for the formation of offshore structures have gradually moved to cold regions, such as the arctic, containing abundant petroleum resources below the seafloor, owing to resource depletion in relatively warm regions. Therefore, it is difficult for the existing high-strength steel sheets having superior toughness at low temperatures to endure an extreme low temperature environment that is severe as above.
Moreover, since thick steel sheets may be used for multipurpose tanks to store and transport liquefied gases having very low liquefied temperatures therein, the thick steel sheets should have a proper degree of toughness, even at a temperature lower than the temperature of the liquefied gas. For example, since the liquefied temperatures of acetylene and ethylene are −82° C. and −104° C., respectively, a high-strength steel sheet having superior toughness when exposed to such a temperature is required.
To secure a toughness required of steel sheets used for tanks, methods of controlling microstructures by adding 6 to 12% by weight of Ni or performing a treatment, such as quenching, tempering, or the like have been used, but such methods have limitations, such as a high manufacturing costs, and a low productivity.
In terms of low carbon steel, while existing steel sheets have superior toughness at a low temperature of about −60° C., it may be difficult for existing steel sheets to satisfy the requirements for steel sheets having superior low-temperature toughness, considering the extreme low temperature environments faced by ships, offshore structures, and the like. Therefore, it may be said that studies into high-strength steel sheets capable of securing superior toughness at extreme low temperatures lower than −60° C. are strongly required.